1. Field of the Invention
This invention relates in general to burners for use in furnaces, boilers, fired heaters and other combustion apparatus for industrial applications. More particularly, the invention relates to low NOx burners and combustion processes for use in such applications.
2. Description of the Related Art
In surface burners, premixed fuel and air burn close to the surface of a porous matrix, fibrous matrix or channeled surface element, with radiative heat loss from the surface reducing the peak flame temperature and NOx. Examples of this type of surface burner are described in Morris U.S. Pat. No. 4,889,481 and Otto U.S. Pat. No. 5,077,089. Burners of this type are limited in their firing rates. As the firing rate is increased, the flame moves away from the surface and this decreases heat transfer back to the surface and thereby decreases radiative heat loss from the flame. Under these conditions, flame temperature and therefore NOx production increase. The limited surface firing rate needed to maintain low NOx limits the application of such burners. Recently, higher firing rate surface burners have been developed, such as described in Duret U.S. Pat. No. 5,439,372, but their NOx emissions are high. To maintain low NOx, these burners must be operated at high excess air, but that in turn reduces their fuel efficiency. The loss in efficiency results because higher excess air leads to higher flue heat losses for a given flue temperature. Secondary fuel injection is also known in the art for low NOx emissions, as for example Johnson U.S. Pat. No. 5,201,650.
The need has therefore been recognized for a low NOx burner and combustion process which obviates the foregoing and other limitations and disadvantages of prior art burners and processes. Despite the various burners and processes in the prior art, there has heretofore not been provided a suitable and attractive solution to these problems.